(a) Field of the Invention
The present invention relates to a variable focal length lens system and, more particularly, to a variable focal length lens system arranged to be capable of photographing with at least two different focal lengths by changing over the focal length by moving a lens unit thereof.
(b) Description of the Prior Art
Video cameras did not become so popular because they were high-priced and heave compared with conventional cameras for silver salt films. Recently, however, video cameras have been made smaller in size, lighter in weight and lower in price, and there is a quickly increasing demand at present for video cameras for general consumers (not for business use). To further widen the scope of users, it is now earnestly required to provide video cameras which are still smaller in size, ligher in weight and lower in price. On account of the above-mentioned requirement, it is of course required to make the lens systems for video cameras also smaller in size, lighter in weight and lower in price. Under existing circumstances, however, lens systems for video cameras are not yet made satisfactorily compact compared with electric systems of video camera bodies.
As for lens systems, it is considerably difficult under existing circumstances to make them still smaller in size, lighter in weight and lower in price by ensuring satisfactory specifications such as the zoom ratio, F-number, etc. unless an epoch-making invention is made in the field of optical elements. Therefore, as a way for solving the above-mentioned problem, there is an idea to provide a lens system with fixed focal length to a video camera and to make the video camera as a whole extremely small in size, and video cameras with such specifications are actually provided already. However, the lens systems with fixed focal length have a disadvantage that the available scope of variety in photographing is narrow.
Such being the cirumstances, attention is called to a focal length change-over type variable focal length lens system which exists between zoom lens systems and lens systems with fixed focal length.
As lens systems to be used with still cameras for silver salt films, there exist a plural number of known focal length change-over type variable focal length lens systems. For example, one of said known lens systems is arranged that a lens unit having negative refractive power is to be inserted on the image side of a lens system with a short focal length so as to thereby change over to the state of a long focal length. Another example of said known lens systems is arranged as a zoom lens system comprising two lens units, i.e., a front lent unit and a rear lens unit.
In case of the former example of the known lens system, the F-number is multiplied by the vari-focal ratio when changed over to the state of the long focal length and, consequently, the lens system becomes dark. Moreover, the vari-focal ratio itself cannot be made higher than about 2 from the view point of imaging performance, and this is not sufficient for satisfying the requirement.
In case of the latter example arranged as a zoom lens system comprising two lens units, when it is arranged to vary the focal length by keeping one of the front and rear lens units fixed and moving the other lens unit only, focal lengths at which the image surface comes to the same position scarcely exist within the vari-focal range and, when it is attempted to arrange that the image surface comes to the same position when the focal length is varied, the performance tends to become unfavourable. Therefore, it is unavoidable to move both of the two lens units in most cases, and it is often necessary to move the stop. Moreover, in the position of the long focal length, it is difficult to make the F-number small, i.e., to make the lens system bright, by keeping the performance in the favourable state and, even when an aspherical surface is adopted, it is difficult to obtain a satisfactory F-number. Furthermore, the vari-focal ratio becomes about 2, and this is not satisfactory.
In cases of the above-mentioned examples of known lens systems, the number of lenses constituting the lens system is six to eight in both cases.
As described so far, as a lens system for a video camera, there exists no known variable focal length lens system of the focal length change-over type which is arranged to be compact, low in price and excellent in performance.